1. Field of the Invention
The invention relates generally to gas generating pellets or tablets capable of generating nitrogen gas at relatively low temperatures on the order of 200.degree. to 1000.degree. F. upon ignition to provide inflation for air bag passive restraint systems. More particularly this invention relates to an improved wet process and apparatus for processing the various make-up ingredients and fabricating pellets or tablets therefrom, along with the resulting improved products.
Though the propellant of this invention is especially designed and suited for creating nitrogen for inflating passive restraint vehicle crash bags, it would function equally in other less severe inflation applications, such as aircraft slides, and inflatable boats, and, more generally, find utility for any use where a low temperature, non-toxic gas is needed, such as for a variety of pressurization and purging applications, as in fuel and oxidizer tanks in rocket motors, for various portable and military equipment and operations where a storable source of nitrogen is desirable, for many laser applications and in outer space stations and outer space vehicle atmospheres where a source of nitrogen is needed, for example, to dilute oxygen.
2. Description of the Prior Art
The use of protective gas-inflated bags to cushion vehicle occupants in crash situations is now widely known and well documented. In early systems of this type, a quantity of compressed, stored gas was employed to inflate a crash bag which, when inflated, was positioned between the occupant and the windshield, steering wheel and dashboard of the vehicle. The compressed gas was released by the action of actuators or sensors which sense a rapid change in velocity of the vehicle during a rapid impact, as would normally occur during an accident.
Because of the bulk and weight of the compressed gas apparatus, its generally slow reaction time and attendant maintenance difficulties, stored gas systems have largely been superseded by systems utilizing a gas generated by chemical gas-generating compositions. These systems involve the use of an ignitable propellant for inflating the air cushion, wherein the inflating gas is generated by the exothermic reaction of the reactants which form the propellant.
The bags used in a restraint system of this type must be substantially inflated within a very limited time span, generally on the order of tens of milliseconds, to accomplish their purpose. In addition, the gas thus produced should meet several rather stringent requirements. As for example, the temperature of the gas as generated should be low enough so as not to burn the bag, undermine its mechanical strength, or burn, or injure the affected passenger in the vehicle in the event the bag ruptures. Also the composition of the gas used in air bag systems should also be non-toxic, non-noxious, non-corrosive, containing very minute amounts of CO, CO.sub.2, NO and NO.sub.2 and less than about 8% H.sub.2 O, and one which is easily filterable to remove solid or liquid particles thus precluding injury to the vehicle occupants and bag damage.
In air bag systems such as those described above, which utilize an ignitable propellant, the stability and reliability of the propellant composition over the life of the vehicle are also very important. Generally, the propellant composition must possess sufficient stability to temperature, humidity and shock so that it is stable and virtually incapable of being ignited except upon deliberate initiation by activating sensors employed for this purpose.
It follows then that the most desirable atmosphere inside an inflated crash bag would correspond in composition to the air outside it. This has thus far proven impractical to attain. The next best solution is inflation with a physiologically inert or at least innocuous gas. The one gas which possesses the required characteristics and which has proven to be the most practical is nitrogen.
The most successful to date of the prior art solid gas generants of nitrogen that are capable of sustained combustion have been based upon the decomposition of compounds of alkali metal, alkaline earth metal and aluminum derivatives of hydrazoic acid, especially sodium azide.
Typical of such prior art which include sodium azide as one of the reactants compositions capable of generating pure nitrogen for airbag applications are the following U.S. Pat. Nos. 3,741,585; 3,775,199; 3,883,373; 3,895,098; 3,920,575; 3,931 040; 3,996,079; 4,062,708; 4,092,190; 4,203,787; 4,369,079; 4,376,002; 4,533,416; 4,547,235; 4,604,151; 4,734,141; 4,758,287 and 4,836,255.
The disclosures in these documents, particularly as it relates to the wide range of azides possible as well as the complimentary ingredients useable in concert therewith and the various mixture formulations thereof, are incorporated herein by reference.
As indicated in aforementioned U.S. Pat. No. 4,369,079 there are problems and disadvantages, however, in the use of these azides, particularly as it relates to the airbag system's utility. Sodium azide, a Class B poison, is a highly toxic material. It is easily hydrolyzed, forming hydrazoic acid which is not only a highly toxic and explosive gas, but it also readily reacts with metal ions such as Ca, Mg, Pb, Fe, Mn and Cu to form extremely sensitive azide compounds that are subject to unexpected ignition or detonation. Special handling in the manufacture, storage and eventual disposal is therefore required to safely handle such materials.
In the past the powdered ingredients making up the various nitrogen producing gas generant compositions were simply dry mixed or blended together with a conventional dry powder blender/mixer until a homogenous mixture was formed, and the resulting mixture then compacted, molded or pelletized into tablets, pellets or granules by conventional techniques using standard equipment, as indicated in aforementioned U.S. Pat. Nos. 3,741,585; 4,203,787 and 4,547,235. And for safety considerations as with most, if not all, pyrotechnic substances, remote controlled tableting presses are convenient devices which maybe employed for compression to tablets. Wet blending and granulation techniques for mixing the azide and oxidant components prior to being compressed into tablets or pellets in the usual manner have also been suggested, as indicated in aforementioned U.S. Pat. Nos. 3,920,575; 3,996,079; 4,376,002; 4,533,416; 4,734,141 and 4,758,287, especially for safety reasons. Of particular note is the '575, '416 and '287 patents wherein at least two solid gas generant reactants, including an azide, are blended with a liquid dispersant (e.g. H.sub.2 O) to form a paste or slurry, which is dried and molded into some predetermined shape.
The instant assignee, Morton International, Inc., has earlier developed a completely automated, (remote controlled) continuous wet process and system (as generally depicted in FIG. 1) for making gas generant tablets or pellets wherein known solid ingredients of a generant azide (e.g. sodium azide) and reactants therefor (e.g. molybdenum disulfide and sulfur) are added to and slurried in water, subjected to wet grinding, spray dried to a powder material, and further processed (e.g. compaction molded) to produce pellets or tablets in the usual fashion.